A unique and important characteristic of all smooth muscles (mammalian and invertebrate) is the ability to vary the energy cost of force production and maintenance during the time course of isometric contractions. The basic mechanisms controlling force maintenance, or tone in smooth muscle are not well understood. Our discovery that the phosphorylation state of twitchin (a mini-titin) regulates catch and force production in the anterior byssus retractor muscle (ABRM) of Mytilus edulis, has opened the way to new studies described here on the molecular basis of its function. Our overall goal is to determine the mechanisms whereby twitchin controls actin-myosin interaction and resulting mechanical output. The ubiquitous presence of twitchin in invertebrate striated, smooth catch and phasic muscles suggests that twitchin may serve as a regulator in all of these muscle types. The understanding of the basic mechanisms underlying this regulation may very well provide new insights on the control of other muscle types, especially mammalian smooth muscle, which shows very similar mechanical characteristics as the smooth muscles from invertebrates. In the proposed studies the smooth ABRM of M. edulis, and striated adductor of the sea scallop (P. magellanicus) will serve as experimental models. Intact and permeabilized invertebrate muscles will be used, as needed. The Specific Aims are to (1) Determine the mechanism by which twitchin phosphorylation gives rise to an increase in the detachment rate constant of the myosin crossbridge; (2) Determine the relationship between the degree of phosphorylation of twitchin and its mechanical effect in intact and permeabilized ABRM; (3) Test the hypothesis that the intrinsic rate of actin movement by myosin is attenuated by the presence of twitchin, and that the phosphorylation of twitchin restores the intrinsic fast rate, using in vitro motility studies of actin movement on native thick filaments from ABRM and the body wall of C. elegans. Nucleotide turnover on the thick filaments will be measured in order to learn how twitchin phosphorylation alters the ATPase activity when calcium concentration is varied. (4) Determine of the effect of twitchin phosphorylation on fast striated scallop adductor muscle in order to learn whether this is a generalized mechanism for the modulation of crossbridge kinetics in invertebrate muscle. (5) Biochemical studies on twitchin will characterize the twitchin molecule and determine the mechanism by which its phosphorylation by protein kinase A controls the interaction of contractile proteins.